Although professional photographers have made available to the general public color pictures of underwater subjects of the most striking and appealing hues, extending over the full range of the visible spectrum, it is much more difficult to obtain pleasing color balances underwater than in photographing above-water subjects. While professional photographers in general prefer color reversal films to color negative films, obtaining pleasing color balances underwater with color reversal films is particularly difficult. In color reversal photography the image captured by the camera is directly viewed after photographic processing, whereas in color negative photography (i.e., negative working films that are printed on color paper) the opportunity exists to compensate for color imbalances in the printing step.
The color balance problem in underwater photography arises from the marked attenuation of red light transmitted through water. Within each 3 meters of light transmission in water approximately half of the red light is absorbed. This results in underwater scenes having a marked cyan cast, indicative of red light deficiency.
The cyan cast of distant underwater subjects is not objectionable, since we have become accustomed to this color balance for distant underwater subjects. This is how they are "supposed" to look. However, there are a large array of foreground subjects for underwater photography that exhibit brilliant colors. Photographs that portray these foreground subjects with a cyan cast that diminishes the brilliancy of the hues, particularly reds and yellows, are understandably judged by most viewers as decidedly inferior. For the professional photographer a picture of a subject with a cyan cast is frequently not marketable and for the amateur the same picture is often a disappointment.
Photographers have employed a variety of techniques for getting brilliant subject hues underwater. The simplest one is to work in shallow water, but with one half red light attenuation occurring in only 3 meters of transmission this is highly limiting. A subject immersed at a depth of 1.5 meters and at a distance from the underwater photographer of 1.5 meters exhibits a one stop (0.3 log E, where E is exposure in lux-seconds) deficiency in red exposure when photographed with a color film exhibiting a daylight (i.e., a 5500.degree. K. color temperature) color balance.
Notice that half of the light transmission distance underwater in the example above is from the atmosphere to the subject and that the remainder of the transmission distance is from the subject to the camera. The photographer can improve the color balance of an underwater subject by moving closer to the subject, thereby reducing the light transmission distance underwater to little more than that required by the depth of the subject. This strategy, however, limits the photographer to approachable subjects. It requires little reflection to realize that successful underwater photography of elusive subjects is often frustrated by the limited light transmission distances that are compatible with capturing brilliant subject colors.
A conventional strategy for correcting color imbalances underwater is to employ one or a combination of camera lens filters. Needler et al U.S. Pat. No. 3,752,670 and Kreutzig U.S. Pat. No. 4,542,929 illustrate filters for underwater photography. Unfortunately, the basic problem is red light deficiency underwater and no lens filter can increase the amount of red light available. Lens filters instead capture blue and green light to bring the red, green and blue light closer to their desired balance. This has the effect of decreasing the light available for image capture and requires compensation by increasing the camera lens aperture and/or decreasing the camera shutter speed. Increasing the camera lens aperture reduces image sharpness and depth of field. Decreasing shutter speed reduces the opportunity for capturing sharp images of moving subjects and raises the risk of image unsharpness attributable to camera shake. The common technique of tripod exposure is considerably more difficult to implement underwater.
A common alternative is to synchronize camera exposure with a daylight (5500.degree. K.) strobe. Maeno U.S. Pat. No. 4,653,883 illustrates an underwater camera and strobe combination. With strobe illumination the light attenuation distance is twice the distance to the subject and independent of the depth, since the light travels from the strobe to the subject and back to the camera. While strobe illumination is potentially disturbing to animate underwater subjects and increases the equipment that must be transported, it often offers the best available approach to photographing near subjects. However, since light must still travel twice the distance of the subject from the photographer, cyan overcast of intermediate distance subjects remains a problem. Notice that at a subject distance of only 1.5 meters (3.0 meters of light transmission distance) half of the red light emitted by the strobe is attenuated by the water before reaching the camera.